Synchronous transmission systems

ABSTRACT

In a digital telecommunications system having data packages carrying identification data, the identification data is characterized in that it is unique to the package within a station and does not depend on the identity of the originating station.

[0001] The present invention relates to telecommunication systems andmore particularly to digital hierarchy systems (SDH). In such systemsdata is switched within an individual station to the appropriate outputport. Also alternate routes are provided between stations by which datacan be transmitted from an originating stations port to a receiverstations port so that failure of one route, e.g. due to accidentaldamage, will not prevent the data from reaching the receiver stationsport via the other route.

[0002] The data is sent in the form of two packages or so-called virtualcontainers which essentially are made up of two sections, one being theactual data which is transmitted, i.e. the so-called payload, and theother being data which is concerned with the integrity of the data,referred to hereinafter as overhead or ancillary data.

[0003] One of the requirements for a system of the kind outlined aboveis that there must be some means for checking whether the package iscorrectly switched (or routed) at each individual station along theoverall path of the data. Essentially this is achieved by use of theancillary or overhead data area at each station in order to include anidentity, and other monitoring information, to ensure correct switching.

[0004] In our co-pending application no. 9301575 we disclose anarrangement where the package is identified in terms of the channelnumber of the data, and the receiving stations card number. Howeverthere is a potential problem with such an arrangement in situationswhere the package travels by the alternative route referred to earlier.In the event of failure of the first route the package will acquire anew channel and card number. This means that the destination port of thestation will not recognise the package because it will be looking for apackage which has an identification which is characteristic of the firstroute. This problem is a direct result of the use of the alternate routeprotection identified earlier.

[0005] The present invention is concerned with overcoming this problem.

[0006] According to the present invention in a digitaltelecommunications system of the kind referred to, data packages carryidentification data which is unique to the package within a station, anddoes not depend on channel or card numbers.

[0007] In designing systems of the kind referred to there is always theconflicting requirement of on the one hand making maximum use of thetransmission capacity to transmit payload data and on the other handensuring the correct transmission of that payload data. In other wordsin each data package a balance has to be struck between the ratio ofpayload capacity and overhead or ancillary capacity.

[0008] Further design constraints are the internationally agreedstandards and protocols which are necessary to ensure that the variousnational telecommunications systems can communicate with one another,e.g. the so-called CCITT standards and protocols.

[0009] Included in the ITU (formally CCITT) standards concerning theabove mentioned overhead or ancillary data is one which calls for datawhose purpose is to provide monitoring of the actual data, referred toas the V4 byte. This is only valid between stations, allowing itseparate use within a station. In addition some data packages (orpayloads) do not have such an overhead that may be used, in which case apart of the Section Overhead of the traffic is used in its place. Forclarity only the V4 byte is described below, but the same principleapplies to the Section Overhead.

[0010] According to one aspect of the present invention overhead orancillary data characteristic of the identification of a data package issubstituted for the so-called V4 byte to ensure the correct routing ofthe data package whilst at the same time not creating an increase in theoverhead or ancillary data contained in the data package. In other wordsthe inclusion of such identification data does not result in arequirement for increased bandwidth.

[0011] More specifically where an add-drop multiplexer (ADMUX) isemployed to switch data from a data transmission channel into aso-called tributary channel the package identification data issubstituted for the V4 byte as the package enters the ADMUX and isre-substituted as the package leaves the ADMUX.

[0012] The package identification data may have the characteristicsreferred to in our co-pending application number 9301575 or thecharacteristics referred to above in connection with the presentinvention.

[0013] How the invention may be carried out will now be described by wayof example only and with reference to the accompanying drawings inwhich:

[0014]FIG. 1 is a diagrammatic representation of a typical transmissionloop connected to a main transmission path; and

[0015]FIG. 2 shows diagrammatically and in more detail one of the ADMUXshown in FIG. 1.

[0016] A transmission loop 2 is connected to a main transmission path 1through a switch in ADMUX 3. There is included in the transmission loop2 ADMUX switches 4, 5 and 6 each of which serves tributaries 7, 8 and 9respectively.

[0017] Data destined for tributary 9 can reach the ADMUX 6 eitherthrough the path 3,4 or through the path 3,5. Data is sent on bothpaths, with the terminating ADMUX (ADMUX 6) selecting which to use. Thisprovides protection against failure of either path. When the packet ofdata reaches its destination at 9, there must be means to check that thecorrect packet of data has been switched down to it by the switch inADMUX 6. The present invention is concerned with ensuring that this isthe case in arrangements where the so-called path protection isprovided, i.e. the data can reach its destination by one of at least tworoutes.

[0018] Referring to FIG. 2, the ADMUX 6 (shown in dotted lines) consistsessentially of four cards 10, 11, 12 and 13. Card 11 is the switchitself. Cards 10 and 12 are so-called synchronous transport modules(STM) which can be designed to multiplex at different levels e.g. 1, 4or 16 which equate to bit rates of 155.52 Mbit/s, 622.08 Mbit/s and2488.32 Mbit/s respectively would thus be designated STM1, STM4 or STM16tributary card.

[0019] Data packages entering the ADMUX 6 at A and B will have insertedinto the ancillary or overhead data in place of the V4 byte, data whichuniquely identifies that data package within that ADMUX. This contrastswith the arrangement disclosed in our co-pending application no. 9301575where the data package is identified in terms of its entry point intothe ADMUX, for example by means of its channel number and card number.This would result in the packages at A and B having differentidentifications.

[0020] This identity is distinct from the other data which forms theoverhead and which is designed to enable various checks to be made inorder to monitor the integrity of the data being transmitted.

[0021] Having described the inventive concept illustrated in broad termsin relation to the accompanying drawings there will now be given a moredetailed description of the embodiment of the present invention.

[0022] It is necessary to ensure that switch protection takes placeautomatically a failure condition at a destination port is detected oralternatively on demand from the ADMUX controller.

[0023] There are a number of conditions which can be used to initiateswitching and these are:

[0024] mismatch on path identification number

[0025] failure of parity checks

[0026] failure of alignment

[0027] loss-of system-clock transitions

[0028] The first two-of the above involve the monitoring of diagnosticmessages inserted by sender ports which are compared with data which isdownloaded from a Mux Controller as a ‘comparison message’ or which iscalculated internally (parity errors). For TU-1 and TU-2 SDH signals,where the diagnostic messages are contained in the V4 bytes and arecompletely updated only every 16 frames (4 V4 bytes), the comparison iscarried out every four frames on a partial V4 message. For TU-3, AU-3and AU4 signals, the diagnostic message is fully updated every frame.

[0029] According to the invention a path is first set up, and acontroller is arranged to generate a 16 bit number which uniquelyidentifies that path, this number is inserted at each sender port intothe diagnostic message for that channel, a separate number being usedfor each unidirectional path (i.e. a bidirectional path utilises 2numbers). It will be appreciated that the 16 bit number generated,allows up to 65536 unique path numbers the ADMX4 being capable of havingup to 1008 bidirectional or 2016 unidirectional cross connections.

[0030] At each destination port, the path identity number of eachchannel, inserted by the sender port, is arranged to be compared withthe expected address (downloaded from the Mux Controllerconfiguration-data).

[0031] The address check is arranged to be carried out on traffic fromboth Switch units simultaneously, to detect failures of either switch.This method of failure detection is preferably subject to a persistencecheck and protection switching is in accordance with establishedpriorities.

[0032] In carrying the invention to effect parity checks are carried-outon the data of each channel. Parity bits are calculated at the senderport and inserted into the diagnostic messages such that even parity ismaintained in each channel. At the destination port, the parity of eachchannel is again calculated and any parity errors generate an alarmsignal. As with the source address checks, the parity checks are carriedout on traffic from the working switch and the standby switchsimultaneously. This method of failure detection is subject to apersistence check and protection switching is in accordance with theallocated priorities.

[0033] For TU-1 or TU-2 signals, V4 bytes occur every fourth frame (i.e.every multiframe) and a complete V4 diagnostic message requires foursuch bytes. Each parity bit for a channel is relevant to all bits fromthat channel within the previous multiframe (four frames), including theV4 byte. The four parity bits of the V4 message are therefore completelyindependent of each other and are related to the data of the previousfour frames only.

[0034] For TU-3, AU-3 and AU-4 signals only eight parity bits arecontained within the diagnostic message within the Signal path overhead(SOH). Since a complete diagnostic message is transmitted every frame,these are used to indicate the parity of the data within the previousframe.

[0035] Each peripheral card incorporates transition-loss detectors whichthe internal clock signals from each switch. A failure of the (38.88MHz) clock shall be defined as a loss-of clock transitions for a periodexceeding a nominal threshold of 125-500 nS. There need not bepersistency check on clock failures.

[0036] A check is necessary to identify any alignment failure. If thealignment bits contained within the diagnostic messages are incorrect,the detecting destination port is arranged to generate a loss-of-framealarm. This check is subject to a persistence check as described later.

[0037] A request to switch is arranged to be issued to all destinationports on receipt of a switch-failure indication by the Mux. controller.A manual request may also be issued when demanded by an operator via theNMI. These requests are treated as switch-failure conditions and actedupon in accordance with prescribed routines. No persistence check iscarried out on such conditions.

[0038] In cases of diagnostic message failure, a persistence check isperformed to prevent spurious switching during transient conditions(e.g. MSP switching). This check takes the form of a threshold on thenumber of permissible consecutive mismatches of the diagnostic messages,with the messages being partially updated and compared every multiframe.A hardware persistence check is therefore applied to the channelidentification number and the parity check independently. Thispersistence is programmable from between 1 and 256 successive failuresof the check.

[0039] The enabling and disabling of protection is carried out accordingto the following criteria:

[0040] a. Unconnected—no checks performed

[0041] b. Connected—all checks performed

[0042] c. Protected—all checks performed

[0043] The V4-byte diagnostic message format is as shown in thefollowing table. MSB Parity Alignment bits bits. LSB V4 byte 1 P1 0 A1 0D3 D2 D1 D0 V4 byte 2 P2 0 A2 0 D7 D6 D5 D4 V4 byte 3 P3 0 A3 0 D11 D10D9 D8 V4 byte 4 P4 0 A4 0 D15 D14 D13 D12

[0044] These bits are defined as follows:

[0045] Data Parity

[0046] Pi P2 P3 P4

[0047] Alignment Bits

[0048] A1 A2 A3 A4

[0049] Path Identification Number

[0050] DO to D15

[0051] The various Section Overhead diagnostic message configurationsare treated as follows:

[0052] TU-3 and AU-3

[0053] The diagnostic message is contained in the first three columns ofthe first three rows of the SOH, as follows. Section Overhead Bytes RowColumn 1 2 3 1 AM1 AM2 AM3 2 BM1 BM2 BM3 3 CM1 CM2 CM3

[0054] Where the bytes shown have the format: MSB LSB AMN = P1 P2 P3 P4P5 P6 P7 P8 BMN = D7 D6 D5 D4 D3 D2 D1 D0 CMN = D15 D14 D13 D12 D11 D10D9 D8

[0055] AU-4

[0056] The diagnostic message is contained in the first column of thefirst three rows of the SOH.

[0057] The three bytes ‘A’, ‘B ’ and ‘C ’ have the same format as bytes‘AMN’, ‘BMN ’ and ‘CMN’ for TU-3 and AU-3 signals.

[0058] Comparison messages sent from the mux. controller and stored inan ASIC at the destination ports have the following format: MSB LSB Byte1 = D7 D6 D5 D4 D3 D2 D1 D0 Byte 2 = D15 D14 D13 D12 D11 D10 D9 D8

[0059] The system described above gives an effective protectionarrangement for ADMX switch plane protection without any need fordisabling a card when protection is applied to a traffic path.

1. In a digital telecommunication system having data packages carrying identification data, the identification data is characterised in that it is unique to the package within a station and does not depend on the identity of the originating station.
 2. A system as claimed in claim 1 which comprises: a) means to set up a uni-directional route or path through the switch; b) a controller to generate an identification number which uniquely identifies that route or path; c) means to insert that identification number at each sender port into a diagnostic message for that channel; and d) means at each of a plurality of destination ports to compare the actual identification number received with that expected.
 3. A system as claimed in claim 2 in which the unique identification number is applied to the data package as it enters an ADMUX, in place of the V4 byte normally carried by the data package, and is replaced by the V4 byte as the data package leaves the ADMUX.
 4. A system substantially as hereinbefore described with reference to and as shown in the accompanying drawings. 